Beam-steering antennas are used in a wide variety of applications. For example, a phased-array antenna can steer one or more radar beams to a desired angle in two dimensions (e.g., azimuthal angle and elevation angle), and can re-steer those one or more beams electronically in microseconds. This electronic beam-steering capability is useful for many applications in radar such as navigation, surveillance, and imaging.
More recently, researchers have developed holographic-aperture beam-steering antennas, which can consume significantly less power than phased-array antennas, and can also be significantly smaller (e.g., significantly thinner and significantly lighter). For example, the dimensions of a square holographic beam-steering antenna can be on the order of 10λ0×10λ0×0.1λ0, where λ0 is the wavelength at which the holographic antenna is designed to operate; therefore, a holographic beam-steering antenna designed to operate at 20 GHz can have dimensions on the order of 15 centimeters (cm)×15 cm×1.5 cm (approximately 7 inches (in)×7 in ×0.7 in).
Ideally, researchers would like to design a holographic-aperture beam-steering antenna that would concentrate all of the signal energy in the main beam(s) at every possible steer angle such that no side beams (hereinafter “side lobes”) would exist (i.e., all side lobes would have zero energy).
But because designing such an ideal antenna, at least with today's technology, is not possible due to, e.g., practical limitations on manufacturing tolerances, and on how small the antenna elements and the inter-element spacing can be, today's holographic beam-steering antennas have multiple side lobes of non-zero energy.
Unfortunately, the smaller the difference(s) between the gain(s) of the main beam(s) and the gain(s) of the major side lobe(s), the lower the signal-to-interference ratio (SIR) of the antenna. Hereinafter, this difference (or these differences if more than one) is referred to as the side-lobe level, and can be an absolute difference, e.g., between the main beam (the smallest main beam in an application with multiple main beams) and the largest side lobe, can be an average difference between the main beam (the smallest main beam in an application with multiple main beams) and each of the side lobes, or can be an average difference between the main beam (the smallest main beam in an application with multiple main beams) and each of the major side lobes. The side-lobe level is said to be improved when its magnitude is increased. That is, if the side-lobe level is calculated, in general, as main beam minus side lobe, then the side-lobe level improves as the side-lobe level increases. Conversely, if the side-lobe level is calculated, in general, as side lobe minus main beam, then the side-lobe level improves as the side-lobe level decreases.
Power in the side lobes is additionally undesirable, as it parasitically increases the total power which the antenna must transmit/receive for a given main-beam characteristic (e.g., gain).
Consequently, researchers are searching for ways to increase the magnitude of the side-lobe level (i.e., to decrease the side-lobe gain) generated by a holographic-aperture beam-steering antenna so as to increase the SIR of the antenna system, and to decrease the overall transmit/receive power required by the antenna.